This invention relates to a solar operated charging device for a rechargeable accumulator, more particularly for small electrical apparatuses such as, for example, electric dry-shavers whose energy is supplied by rechargeable accumulators which can be charged by means of a charging device comprising solar cells.
Such a charging device is known from DE-OS 36 41 978. The outer side of a dry-shaver is provided with a solar cell module which is directly connected to the accumulators. In order to obtain a sufficient charge current, the solar cell surface is dimensioned as large as possible, particularly by folding out additional sheets of solar cells. However, particularly when nickel-cadmium accumulators are used, the direct charging is difficult because the currents supplied by the solar cell module in the case of cloudy weather or in winter become so small that they fall below the minimum charge current values (charge current threshold) of approximately one thirtieth of the accumulator capacity as stated by the accumulator manufacturers. For example, a minimum permanent charge current of 20 mA is permissible at a capacity of 600 mAh of a nickel cadmium accumulator. Smaller accumulator capacities, which could be operated with smaller charge currents, are not suitable because the accumulator would run down too fast if it were not recharged.
The solar current supplied by the solar cell module depends on the active surface area of the solar cells and on the amount of sunshine. The surface area of the solar cell module is limited by the dimensions of the apparatus so that a minimum value can be determined for the amount of sunshine which can be collected. If the active surface area is approximately 40 cm.sup.2, the required minimum solar radiation for currently used solar cell modules is approximately 36,000 Lx for charging these accumulators at a rated voltage of 2.4 V. Dependent on cloudiness, location and light transmissiveness of the window glass, this value is not achieved for longer periods during the winter season so that the apparatus cannot be recharged at all because the currents supplied by the solar cell module are too small. On the other hand, the accumulator could be supplied with the energy for one shave, i.e. approximately 360 Ws, in approximately 4 hours by means of a current of 10 mA. Even smaller currents would still lead to a longer available shaving time without an additional charge from the mains.
The book "Geratebatterien" by H. A. Kiehne et al, Expert Verlag, Vol. 104, Kontakt & Studium, Elektrotechnik, page 50 states the feasibility of improving the charge capability of accumulators at very small charge currents or of developing new circuits for converting small charge currents into stronger current pulses.